This invention relates to a detector for both metallic and non-metallic buried objects. Previously, metallic underground pipes were used almost exclusively in the transportation of natural gas. Location of the buried metallic pipes was relatively simple since metal reflects high frequency electromagnetic waves which can be easily detected. However, underground metal pipes have inherent problems. They are subject to corrosion to differing degrees, difficult to install and are becoming more difficult and expensive to buy. As a result of these limitations, other types of pipe have become popular. Polymeric pipe being virtually noncorrosive, light, easily installed and relatively inexpensive is rapidly replacing metallic pipe.
A problem with polymeric pipe is that an underground installation cannot be located with conventional metal detectors. As a result, subsurface detectors of non-metallic and metallic objects have evolved. An example of these are shown by Patent Nos.:
Pat. No. Patentee Dated ______________________________________ 2,901,688 W. M. Barret 8/25/59 2,901,689 W. M. Barret et al 8/25/59 2,999,979 H. J. Woll 9/12/61 3,351,936 A. M. Fedor 11/7/67 3,392,384 L. Wesch 7/9/68 3,629,813 H. Drenkelfort 12/21/71 3,728,026 E. Idestrom et al 4/17/73 ______________________________________
Most of them operate by emitting a radar-like signal, reflecting it off of a target, receiving the reflected wave, operating on it and viewing it. The target reflects the waves differently than its environment because of its different dielectric constant. The surface of the ground, minerals and other items have different dielectric constants and produce signals which may give deceptive information. Water content, in particular, varies the dielectric constant substantially and makes consistent detection of targets difficult at best.
Some of the above-noted patents have tried to compensate for the ground effect in different ways. For example, the Wesch U.S. Pat. No. 3,392,384 utilizes a dual antenna system in an attempt to eliminate the effect of variations in the surrounding environment's dielectric constants. The Wesch device uses a signal transmitting device and two spaced antenna receiving systems, one over the target and one over the surrounding environment. The signals from the two receiving antennas are compared in an attempt to isolate the signal from the target. An inherent problem in this system is the critical nature of the dual antennas. If they should, for any reason, become different in their reception characteristics or if their spacing is not correct, accurate detection of a target will be severely hampered.
This invention substantially solves the problem of compensation for variations in the dielectric constant without resorting to a dual antenna system with its accompanying limitations. The use of less equipment permitted the design of this portable unit which is capable of transportation and operation by a single operator. This invention utilizes a single antenna having a transmitting and receiving section and, thus, avoids any problem of matching or spacing antennas. Compensation for different dielectric constants is accomplished by sampling the dielectric constant near the target area and comparing the resultant signal with that received from the target area. The compensation is handled electronically by operating on and storing a first signal in order that it can be recalled for comparison with a second signal received from the target. It is very important that the signals are converted to a digital form in order to facilitate their storage and comparison in a machine which can be easily transported to and used in the field by a single operator. The equipment necessary to operate on an analog signal in a similar manner would be of such magnitude as to be unuseable in the field.